Superconductor incorporating therein superconductivity epitaxial thin film and manufacturing method thereof

ABSTRACT

The present invention utilizes magnesium diboride (MgB 2 ) or (Mg 1-x M x )B 2  as a superconductivity thin film which can be applied to a rapid single flux quantum (RSFQ) circuit. A method for manufacturing a superconductor incorporating therein a superconductivity thin film, begins with preparing a single crystal substrate. Thereafter, a template film is formed on top of the substrate, wherein the template has a hexagonal crystal structure. The superconductivity thin film of MgB 2  or (Mg 1-x M x )B 2  is formed on top of the template film. If Mg amount in the superconductivity thin film is insufficient, Mg vapor is flowed on the surface of the superconductivity thin film while a post annealing process is carried out at the temperature ranging from 400° C. to 900° C.

This is a divisional application of prior application Ser. No.09/994,609 filed on Nov. 28, 2001, now U.S. Pat. No. 6,626,995

FIELD OF THE INVENTION

The present invention relates to a superconductor; and, moreparticularly, to a superconductor incorporating therein an intermetalliccompound superconductivity epitaxial thin film and a method formanufacturing the same, using a superconductor material such asmagnesium diboride (MgB₂) or (Mg_(1-x)M_(x))B₂.

DESCRIPTION OF THE PRIOR ART

In general, a fabrication of a superconductivity thin film has beenadvanced for tens of years for the purpose of an electronic circuitapplication. Particularly, the fabrication of the thin film and itsapplication to the electronic circuit has been mainly researched anddeveloped by utilizing niobium (Nb) of a low temperature superconductorand Y₁Ba₂Cu₃O_(7−x) (YBCO) of a high temperature superconductor, whereina superconductivity transition temperature (Tc) of Nb is 9.2 K and thatof the YBCO is 93 K.

The superconductivity transition temperature (Tc) of a YBCO thin film ishigher than Tc, i.e., 77 K, of liquid nitrogen, and an energy gap of theYBCO thin film is greater than that of the low temperaturesuperconductor so that it may be applied to the electronic circuit witha high speed performance. But, the YBCO thin film has a limitation thata uniform junction is too hard in a manufacturing process so that it isdifficult to manufacture an integrated circuit (I.C).

On the contrary, Nb of the low temperature superconductor has advantagesthat the junction process is easy so that it may be applied to thefabrication of the I.C. However, an operation of the I.C is performed ata temperature below than the superconductivity transition temperature ofliquid helium (He), i.e., about from 4 K to 5 K, so that the Nb thinfilm is less practical in the view point of economy.

In recent years, magnesium diboride (MgB₂) is discovered as thesuperconductor material. The MgB₂ is an intermetallic compoundsuperconductor having magnesium (Mg) and boron (B) therein. Thecomposition of the MgB₂ is relatively simple and the superconductivitytransition temperature is high, i.e., 39 K, so that it can be applied tothe electronic circuit in case of fabricating MgB₂ as the thin film.

If MgB₂ is applied to the electronic circuit, the operation of thecircuit can be performed at the temperature ranging from 15 K to 20 Kusing a conventional cryocooler and the speed of the circuit isapproximately 4 times as fast as that of an Nb circuit which is operatedat the temperature ranging from 4 K to 5 K. In addition, since anoperation temperature of the circuit ranges from 15 K to 20 K, it isunnecessary to use liquid nitrogen so that it may be widely applied toan electronic device economically.

Up to now, several technologies for MgB₂ has been announced such as afabrication technology of an MgB₂ powder, an MgB₂ pellet and an MgB₂wire.

Referring to FIGS. 1 and 2 are graphs setting forth a X-ray diffractionpattern and a relation between the resistivity and the temperature of anMgB₂ thin film in accordance with a first prior art. According to thefirst prior art, the method for manufacturing the MgB₂ pellet beginswith mixing Mg and B to a ratio of 1:2. Thereafter, mixture of Mg and Bis pressurized at a high temperature in a hot isostatic pressing (HIP)furnace, thereby obtaining an MgB₂ pellet having the superconductivitytransition temperature of 39 K. This is disclosed by J. Nagamatsu, N.Nakagawa, T. Muranaka, Y. Zenitani and J. Akimitsu in an article,“Superconductivity at 39 K in Magnesium Diboride, Nature 410, 63, 2001”.

A second prior art for manufacturing the MgB₂ pellet is disclosed by C.U. Jung et al., in an article,“Temperature-and-Magnetic-Field-Dependences of Normal State Resistivityof MgB₂ Prepared at High Temperature and High Pressure Condition,http://www.lanl.gov/cond-mat/0102215”. In a disclosure, MgB₂ isfabricated in a type of pellet under the high temperature and the highpressure by using an anvil-typed press.

Furthermore, a third prior art for manufacturing the MgB₂ powder isdisclosed by S. L. Bud'ko et al., in an article, “Boron Isotope Effectin Supercoducting MgB₂, Phys. Rev. Lett., 86, pp. 1,877-1,880, 2001”.According to the third prior art, to begin with, mixture of Mg and B isinserted into a tantalum (Ta) tube after Mg and B are mixed to a ratioof 1:2. Thereafter, the Ta tube is vacuum-sealed using a quartz capsule.Finally, the Ta tube provided with mixture of Mg and B therein isannealed at 950° C. and then cooled, thereby obtaining the MgB₂ powder.

A fourth prior art for manufacturing the MgB₂ wire is disclosed by P. C.Canfield et al., in an article, “Superconductivity in Dense MgB₂ wire,Phys. Rev. Lett., 86, pp. 2,423-2,426, 2001”. In a paper, boron fiberand Mg are inserted into the Ta tube and the tube is vacuum-sealed usingthe quartz capsule. Thereafter, it is annealed at 950° C., therebyobtaining the MgB₂ wire.

However, it is impossible for the MgB₂ powder and pellet and wirefabricated by the prior arts to be applied to the fabrication of theelectronic circuit. Thus, a fabrication method for the MgB₂ thin film isrequired to be applied to the electronic circuit.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide asuperconductor incorporating therein a superconductivity epitaxial thinfilm of magnesium diboride (MgB₂) or (Mg_(1-x)M_(x))B₂ which can beapplied to a rapid single flux quantum (RSFQ) circuit.

It is, therefore, another object of the present invention to provide amethod for manufacturing a superconductor incorporating therein asuperconductivity epitaxial thin film of MgB₂ or (Mg_(1-x)M_(x))B₂ whichcan be applied to a rapid single flux quantum (RSFQ) circuit.

In accordance with one aspect of the present invention, there isprovided a superconductor comprising: a template film having a hexagonalcrystal structure; and a superconductivity thin film formed on top ofthe template film, including magnesium (Mg) and boron (B) therein whichare epitaxially grown up, wherein a crystal structure and a latticeconstant of the template film are similar to those of thesuperconductivity thin film.

In accordance with another aspect of the present invention, there isprovided a method for manufacturing a superconductor comprising thesteps of: preparing a substrate; b) forming a template film on top ofthe substrate, wherein the template film has a hexagonal crystalstructure; and c) forming a superconductivity thin film on top of thetemplate film having Mg and B therein.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of the preferredembodiment given in conjunction with the accompanying drawings, inwhich:

FIG. 1 is an X-ray diffraction pattern of magnesium diboride (MgB₂)pallet in accordance with a prior art;

FIG. 2 is a graph setting forth a relation between a resistivity and atemperature of the MgB₂ pallet in accordance with the prior art;

FIG. 3 is a schematic view setting forth an MgB₂ thin film or an(Mg_(1-x)M_(x))B₂ thin film in accordance with a first preferredembodiment of the present invention;

FIG. 4 is a schematic view setting forth the MgB₂ thin film or the(Mg_(1-x)M_(x))B₂ thin film in accordance with a second preferredembodiment of the present invention;

FIGS. 5A to 5C are schematic views illustrating an Mg_(1-x)B₂ target inaccordance with the first and the second preferred embodiments of thepresent invention;

FIGS. 6A to 6C are schematic views of an (Mg_(1-x)M_(x))B₂ target inaccordance with the first and the second preferred embodiments of thepresent invention; and

FIG. 7 is a phase diagram of MgB₂ in accordance with the first preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3, there is shown a schematic view setting forth asuperconductor incorporating therein a superconductivity epitaxial thinfilm having magnesium (Mg) and boron (B), in accordance with a firstpreferred embodiment of the present invention. The superconductorcomprises a single crystal substrate 11 having a hexagonal crystalstructure and the superconductivity epitaxial thin film 12 formed on topof the single crystal substrate 11.

Here, one of materials may be utilized as the single crystal substrate11, as described in table 1.

TABLE 1 Material Lattice Constant (nm) Crystal Structure GaN 0.3180hexagonal Al₂O₃ 0.24747 hexagonal SiC 0.3082 hexagonal ZnO 0.3250hexagonal LiAlO₂ 0.3134 tetragonal LiGaO2 0.3186 orthorhombic

Referring to table 1, if the superconductivity epitaxial thin film 12 ismagnesium diboride (MgB₂) of which the crystal structure is thehexagonal structure and lattice constant is 0.3086 nm, the materialhaving the crystal structure and the lattice constant similar to thoseof MgB₂, is preferred to be used as the single crystal substrate 11, forexample, gallium nitride (GaN), aluminum oxide (Al₂O₃), silicon carbide(SiC), zinc oxide (ZnO), LiAlO₂, LiGaO₂. Here, it is noted that LiAlO₂and LiGaO₂ are a tetragonal and an orthorhombic structure respectively.But the lattice constants of LiAlO₂ and LiGaO₂ are similar to that ofMgB₂ so that LiAlO₂ and LiGaO₂ may be used as the substrate for growingup MgB₂.

Referring to FIG. 4, there is shown a schematic view setting forth asuperconductor incorporating therein a superconductivity epitaxial thinfilm having Mg and B, in accordance with a second preferred embodimentof the present invention. The superconductor comprises a substrate 21, atemplate film 22 formed on top of the substrate 21 and thesuperconductivity epitaxial thin film 23 formed on top of the templatefilm 22. The substrate 21 includes a material such as silicon (Si),gallium arsenide (GaAs), metal, magnesium oxide (MgO) and strontiumtitanium oxide (SrTiO₃). The template film 22 uses LiAlO₂, LiGaO₂ or thematerial having the hexagonal crystal structure, wherein the templatefilm 22 is used as a buffer layer or a seed layer. The material havingthe hexagonal structure is selected from the group including GaN, Al₂O₃,SiC and ZnO.

From the first and the second embodiments, it is understood that ifmaterial having the crystal structure and lattice constant similar tothose of MgB₂, is used as the substrate or the template film, MgB₂ isepitaxially grown up with ease.

Instead of MgB₂ thin film as illustrated in the above embodiments,(Mg_(1-x)M_(x))B₂ thin film having the hexagonal crystal structure mayalso be used as the substrate or the template film, wherein M is amaterial selected from the group including copper (Cu), zinc (Zn),sodium (Na), beryllium (Be) and lithium (Li), and x denotes a rationalnumber ranging from 0 to 1.

A method for manufacturing a superconductivity epitaxial thin film ofMgB₂ or (Mg_(1-x)M_(x))B₂, is set forth in detail hereinafter.

To begin with, the MgB₂ thin film is formed by using a method such as asputtering, a pulsed laser deposition, a chemical vapor deposition(CVD), a dual ion beam deposition, an E-beam evaporation or a spincoating technique. In case of using the pulsed laser deposition method,there is an advantage that the thin film having a composition similar tothat of the target can be obtained because the superconductivity thinfilm is deposited after forming Ar-plasma on the surface of thesubstrate.

The CVD method has the advantage that it is appropriate for a massproduction of a large size thin film because stoichiometry of Mg and Bcan be controlled by adjusting the flow of the carrier gas of Mg-organicmaterial and B-organic material. By using the E-beam evaporation and thedual ion beam deposition, it is possible to obtain a high-grade thinfilm in a high vacuum state and further to control the stoichiometry ofMg and B independently.

Since the vapor pressure of Mg is high, a stoichiometric MgB₂ target ornon-stoichiometric Mg_(1+x)B₂ target is used for forming the MgB₂epitaxial thin film by using the sputtering, the pulsed laser depositionor the E-beam evaporation method. At this time, the MgB₂ target is apellet-typed target which is made by pressurizing MgB₂ powder orpressurizing and heating MgB₂ powder at the same time. The Mg_(1+x)B₂target is a sintered material in which Mg is added to MgB₂ powder, amosaic-typed target using an Mg-metal plate and a B-metal plate, or anMg-charged target in which Mg-metal plate is charged. In case ofdepositing by using the Mg_(1+x)B₂ target, the Mg_(1+x)B₂ target plays arole in supplementing insufficient amount of Mg. The Mg_(1+x)B₂ targetis applied to the sputtering, the pulsed laser deposition or the E-beamevaporation method.

Referring to FIGS. 5A to 5C, there are shown schematic viewsillustrating the Mg_(1+x)B₂ target in accordance with the presentinvention, wherein FIG. 5A shows the sintered material in which Mg isadded to MgB₂ powder, FIG. 5B is the mosaic-typed target using anMg-metal plate and a B-metal plate, and FIG. 5C represents an Mg-chargedtarget in which Mg-metal plate is charged.

Referring to 6A to 6C, there are shown schematic views of an(Mg_(1-x)M_(x))B₂ target in accordance with the present invention. The(Mg_(1-x)M_(x))B₂ target is selected from the group including a sintered(Mg_(1-x)M_(x))B₂ which the mixture of MgB₂ powder and M are pressurizedand heated, an (Mg_(1-x)M_(x))B₂ mosaic-typed target using the Mg-metalplate, the B-metal plate and an M-metal plate, an(Mg_(1-x)M_(x))B₂-charged target which M is supplemented therein. Here,M denotes a material selected from the group including Cu, Zn, Na, Beand Li. The (Mg_(1-x)M_(x))B₂ target is used for the method such as thesputtering, the pulsed laser deposition or the E-beam evaporationtechnique.

Referring to FIG. 7, there is shown a phase diagram of MgB₂ inaccordance with the present invention.

In order to fabricate the MgB₂ epitaxial thin film by using the abovemethod, it is necessary to supply heat energy while the thin film isgrown up. Therefore, the substrate is heated up to the temperatureranging from 400° C. to 900° C. where a L₁+MgB₂ phase is formed, asshown in FIG. 7.

Furthermore, it is preferable that the deposition of the thin filmshould be carried out in vacuum state, in argon (Ar) ambient atmosphere,in mixed gas of Ar and hydrogen (H₂) ambient atmosphere and in mixtureof Ar and water vapor ambient atmosphere.

If the Mg amount of the thin film is insufficient after depositing thethin film, magnesium vapor is flowed on the surface of the thin film forsupplementing Mg while a post annealing process is carried out at thetemperature ranging from 400° C. to 900° C. The post annealing processshould be carried out in the atmosphere without oxygen, such as invacuum state, in argon (Ar) ambient atmosphere, in mixed gas of Ar andH₂ ambient atmosphere and in mixture of Ar and water vapor ambientatmosphere. Mg atoms supplemented during the post annealing processdiffuse from the surface of the thin film into an Mg insufficient area,thereby achieving the stoichiometry of MgB₂.

Another method for manufacturing the MgB₂ epitaxial thin film beginswith depositing a boron thin film on the substrate at a roomtemperature. Thereafter, Mg vapor is flowed on the surface of the boronthin film while the post annealing process is carried out at thetemperature in the range of 400° C. to 900° C., thereby obtaining theMgB₂ thin film. Otherwise, an amorphous MgB₂ thin film is deposited onthe substrate at the room temperature and then a passivation film isdeposited on the amorphous MgB₂ thin film. Subsequently, the annealingprocess is carried out at the temperature ranging from 400° C. to 900°C., thereby obtaining the MgB₂ thin film.

In case of depositing the template film as the buffer layer or the seedlayer, the deposition method is used such as the sputtering, the pulsedlaser deposition, the CVD, the dual ion beam deposition, the E-beamevaporation or the spin coating technique.

In conclusion, in case of applying the MgB₂ or the (Mg_(1−x)M_(x))B₂thin film of the present invention to a rapid single flux quantum (RSFQ)circuit, there is an advantage that the operation of the circuit can beperformed at the temperature ranging from 15 K to 20 K using aconventional cryocooler. Furthermore, there is another advantage thatthe speed of the circuit is approximately 4 times as fast as that of anNb circuit which is operated at the temperature ranging from 4 K to 5 K.

Although the preferred embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

What is claimed is:
 1. A superconductor comprising: a template film having a hexagonal crystal structure; and a superconductivity thin film formed on top of the template film, including magnesium (Mg) and boron (B) therein which are epitaxially grown up, wherein a crystal structure and a lattice constant of the template film are similar to those of the superconductivity thin film.
 2. The superconductor as recited in claim 1, wherein the template film is made of a material selected from the group including gallium nitride (GaN), aluminum oxide (Al₂O₃), silicon carbide (SiC), zinc oxide (ZnO), LiAlO₂, LiGaO₂, magnesium oxide (MgO) and strontium titanium oxide (SrTiO₃).
 3. The superconductor as recited in claim 1, wherein the superconductivity thin film is made of a material selected from the group including MgB₂ and (Mg_(1-x)M_(x))B₂, where, M is a material selected from the group including copper (Cu), zinc (Zn), sodium (Na), beryllium (Be) and lithium (Li), and x is a rational number ranging from 0 to
 1. 4. The superconductor as recited in claim 1, further comprising a substrate beneath the template film, wherein the substrate is made of a material selected from the group including silicon (Si), gallium arsenide (GaAs), metal, MgO, gallium nitride (GaN), aluminum oxide (Al₂O₃), silicon carbide (SiC), zinc oxide (ZnO), LiAlO₂, LiGaO₂, magnesium oxide (MgO) and strontium titanium oxide (SrTiO₃). 